Implement Large Eddy Simulation 2D

[wdeconinck]

• Depends on finishing implementation of Navier-Stokes 2D ( issue Implement Navier-Stokes 2D #2 )
• No effort will be spent on subgrid-scale models, as 2D LES is unreal.

Plans:

• Smagorinsky SGS model
• WALE SGS model